Photonic crystals have been drawing attention as a new optical device in recent years. Photonic crystals are optical functional materials having a cyclic refractive index distribution, where a band structure is formed with respect to energy such as light and electromagnetic waves. Particularly characteristic features include formation of energy regions (i.e. photonic band gaps) where propagation of light and electromagnetic waves is impossible.
When refractive index distribution disturbances (or defects) are introduced into the refractive index distribution of photonic crystals, energy levels (or defect levels) caused by the defects are formed in photonic band gaps. Owing to that, light having a wavelength corresponding to energy of a defect level in a photonic band gap is exclusively allowed to exist in the position of the defect. Accordingly, optical circuit elements such as optical resonators composed of point-like defects and optical waveguides composed of linear defects can be provided in photonic crystals. A multiple number of these optical circuit elements are provided in one photonic crystal to compose an optical integrated circuit, where the photonic crystal is made to be an optical IC element. Discrete optical circuit elements are individually connected in the conventional technical field of optical communication or the like, whereas circuit microminiaturization can be realized by using the optical IC elements.
Photonic crystals include two-dimensional photonic crystals and three-dimensional photonic crystal. Three-dimensional photonic crystals have a merit of having little external leakage problem of light existing in defect positions in comparison with two-dimensional photonic crystals. Patent Document 1 describes a three-dimensional photonic crystal having a structure of a plurality of stacked stripe layers where rods made of materials having a higher refractive index than that of air are cyclically arranged in a parallel state from each other, and rods of the most adjacent stripe layers are orthogonal from each other, while rods of the next adjacent stripe layers are disposed in parallel and deviated by a half cycle. This document also describes formation of optical waveguides by providing linear defects in the rods to compose the three-dimensional photonic crystal.
Patent Document 1 further describes a method to manufacture this three-dimensional photonic crystal. The outline is as follows.
First, a planar material layer made of rod materials is formed on a substrate. A stripe structure is formed in this material layer by using photolithography and RIE (reactive ion etching). Two of the material layers attached to the substrates in the same stripe structure formation are superposed in the rod planes so that the rods are orthogonal from each other, followed by heating and fusing them. Thereafter, one of the substrates is removed to obtain a structure where two stripe layers are superposed on one substrate. Two sets of such structures are fabricated and superposed so that the rods in the stripe layers of the uppermost surfaces are orthogonal to each other and the rods in two of the stripe layers with a single layer inserted therebetween are arranged in parallel and deviated by a half cycle, where the uppermost stripe layers are fused by heating. Therefore, a structure with stacked four stripe layers can be obtained. Similar operation is repeated to obtain a three-dimensional photonic crystal composed of the stripe layers of 8 layers, 16 layers, and so on. This method is referred to as “fusion method” hereinafter.
Patent Document 2 describes a three-dimensional photonic crystal where point defects are provided in rods. The point defects include: a defect obtained by arranging a substance having a different shape, refractive index or the like in a partially removed region of a rod; a defect obtained by attaching a member to a rod without removing a part of a rod; a defect obtained by changing a shape of a rod itself (i.e. making it thicker/thinner); and the like. These point defects are made to be resonators which resonate with light of a certain frequency determined by the shape and size of the point defects and the position (or displacement) thereof with respect to the rods. The resonators can be component elements of an optical IC element as described above. Moreover, by introducing a light emitter to the defect portion, the point defect can be used as a laser light source where light is emitted by light resonance in the point defect.    Patent Document 1: Japanese Unexamined Patent Publication No. 2001-074955 ([0007], [0028] to [0035], FIG. 1 and FIG. 8)    Patent Document 2: Japanese Unexamined Patent Publication No. 2004-006567 ([0013] to [0015], FIG. 3 to FIG. 4)